Friend leukemia virus (FLV) is a retrovirus complex which causes a two-stage disease in suseptible mice. The first stage is characterized either by anemia or polycythemia, depending on the virus strain; in the second stage, an aggressive and rapidly fatal erythroleukemia develops. Studies have demonstrated that it is the envelop (env) gene of the spleen focus forming virus (SFFV), a replication defective component of the FLV complex, which is responsible for the first, and probably also the second stage of FLV disease. The other virus of the complex, Friend leukemia virus (F-MuLV) probably serves as a helper virus for replication of SFFV. The precise mechanism by which this complex causes malignancy is unknown. We have produced an antisense clone for the env gene of SFFVA, the anemia inducing strain of SFFV. This antisense gene is driven off the SFFV promoter, which is the retroviral long terminal repeat (LTR). We propose to introduce this antisense gene into the germ line of mice by pronuclear microinjection. Animals will be screened by northern blot hybridization for high expression of the antisense gene, which has the potential to interfere with replication and/or pathogenicity of SFFV. The mice expressing the most antisense RNA will then be challenged, along with controls, with FLV. They will then be observed to determine if they are immune to FLV by virtue of expression of the antisense RNA. Replication of FLV in the animals will be studied in several ways. The number of spleen colonies forming after intravenous injection of virus will be compared to controls. Titers of newly replicated virus in spleens of infected transgenic mice will be compared to controls. The time of development of anemia and erythroleukemia, if these conditions occur, will also be compared to controls. Through these studies we will attempt to determine if antisense constructs to the env gene of this retrovirus, when expressed in the target tissue of retroviral infection, can inhibit viral replication and render animals resistant to disease. This work will complemented by in vitro transfection experiments wherein the antisense gene will be introduced into cells, and the effect of its expression of subsequent replication o FLV after infection assessed.